In two cases of virus induced leukemias in mice we have found that the leukemic stem cells possess malignant growth characters when growing in the spleen of leukemic mice. However, if the same stem cells are grown in the irradiated but otherwise normal, non-leukemic spleen of isogeneic hosts, the leukemic stem cell grows as if it were a normal stem cell. We propose to investigate this phenomenon of leukemic reversibility on Rauscher leukemic stem cells and to isolate the target cells for the leukemic virus, since our experiments strongly suggest that the hematopoietic stem cell is not the primary target of the leukemogenic agent, in contrast to certain assumptions.